The present invention relates to drilling apparatus, and, in particular, to apparatus for use in drilling underground, e.g. drilling under highways for the purpose of installing components such as cables, ducts and pipes.
It is known to use a process called "directional drilling" to install services in sensitive areas where open cut techniques cannot be used such as under runways, motorways, rivers and railways. A small diameter hole is first bored under the crossing using a boring head and drill string made up of steel tubes through which a bentonite slurry known as "Mud" is pumped at high pressure. The boring head is then replaced with a back reamer behind which is connected, via a swivel, the plastic pipe (or other component) to be installed. The drill string is then rotated and pulled back, whilst mud is again pumped through the rods and out of jets in the reamer. The rotating reamer enlarges the hole, whilst the mud cools it and stabilises the opening. The pipe is thus pulled back under the crossing. If, however, the swivel seizes up, the plastic pipe is wound up and irreparably damaged, and the crossing is abandoned, wasting costly time and materials.
A conventional swivel has a tubular body portion into which a swivel shaft extends, there being a bearing to facilitate relative rotation. There is a lip seal intended to exclude mud and other debris from the, bearing. Grease is normally pumped into the swivel before each shot, but because the seal is orientated to keep mud out rather than grease in, this tends to lift the lip of the seal off the swivel shaft and allow ingress of grit. The seal immediately starts to deteriorate and allows more debris to pass. When this reaches the bearing, the eventual destruction of the bearing, and hence the swivel, is inevitable. Common practice is to throw swivels away and replace them before they fail. Thus, an improved sealing arrangement is desirable.
The bearings are also subject to large forces. In order to make them better able to stand up to the forces, larger bearings may be used. But this requires larger housing spaces to be provided in the swivel devices, leading to many practical problems. It is also known to use two or more angular contact bearings, stacked one behind another along the swivel shaft, with a backing nut behind the last one. But this is not very effective.